DATAJPA-269 - Add support for deriving the identifier of nested composit...

...e key entities.

Extended JpaMetamodelEntityInformation to deal with entities with custom composite keys for which we have to derive the identifier. 
Introduced IdentifierDerivingDirectFieldAccessFallbackBeanWrapper that is capable of deriving identifier values.
Added test case to verify the support for save / load of entities with custom composite keys.